Escalating threats of industrial action dominated the Easter conferences of England's biggest teachers' unions, forcing the Government to take a more placatory tone.

Leaders of the National Union of Teachers (NUT) should discover on Monday whether members have supported a one-day strike on April 24. It would be the first national walk-out by teachers for more than 20 years.

At its conference in Manchester, the union voted to ballot its 255,000 members for further rolling strikes if it gained support for the first day of industrial action.

And Ed Balls, the Schools Secretary, faced unrest from the usually more placid NASUWT at the union's conference in Birmingham this week.

Minutes before Mr Balls took to the stage, the 850 delegates voted unanimously to ballot the 250,000 members for national industrial action, beginning in January next year, in protest at extra administrative tasks being unlawfully demanded of teachers without pay.

Earlier, the union had voted to ballot for strikes at schools earmarked to become academies, saying the schools had flouted agreements on pay and conditions.

Chris Keates, NASUWT general secretary, said Ofsted or local authorities should be required to strictly enforce the national workload agreement, to reduce teachers' working hours while rewarding them for additional responsibilities.

A survey of 16,500 teachers commissioned by the union said they had been required to carry out tasks that are legally required to be delegated to support staff, such as record-keeping, bulk photocopying and investigating pupil absences.

Delegates demanded heavy penalties on local authorities that they said were "flouting the law with apparent impunity", and on "complicit" school governors and "arrogant" heads.

Mr Balls said he would ask the social partnership, an advisory body made up of unions, employers and his officials, to look into allegations that teachers were unlawfully being given inappropriate tasks. He said it was "not right" that teachers be required to cover absent colleagues, and promised to crack down on schools that broke the rules.

Until now, ministers have been critical of the threats of strike action. But Mr Balls took a more conciliatory tone in acknowledging the NASUWT vote to ballot for industrial action.

"Because you're professionals, in the end your professionalism can often mean that you work harder and work longer to meet expectations," he said.

Later, he added: "I don't think there's a single teacher or trade union leader in the country who would ever want to take industrial action if it could be avoided, and no parent would want to see the education of their children disrupted."

NUT members have until Monday to vote for or against a single day of strikes over pay, which could now lead to further, rolling industrial action.

Enthusiasm for a strike is evident among London activists, who have already organised a major protest for April 24. But it remains to be seen whether other members will back the action. Some delegates said teachers in other parts of the counry seemed uncertain, and that a low turnout might make it impossible for the union's executive to authorise a national strike.

Teachers are being offered a pay rise of 2.45 per cent from September, with rises of 2.3 per cent for the following two years. But the NUT is continuing the call for above-inflation pay rises of 10 per cent or pound;3,000, whichever is greater.

The union is also asking the Government to commit to a legally binding maximum class size of 20 by 2020, and a moratorium on school closures. The demands were accompanied by angry rhetoric from the executive and the union's general secretary, Steve Sinnott.

"It is fully justified for teachers to take strike action against real-terms pay cuts," he said. "Our campaign is about fairness and self-respect. We must resist attempts to cut the living standards of teachers and their families."

Ian Murch, the union's treasurer, warned Mr Balls to put on his "tin hat" in preparation for the union's planned onslaught. "If I were Mr Brown, I'd be doing my sums again, - cutting our pay is a fraud and a lie," he said.

Unfair pay for supply teachers was also discussed at the conference after it was revealed some schools in Bristol were paying agencies between pound;180 and pound;220 a day for supply staff who only received pound;80.

Later at the NASUWT conference, delegates voted to ballot for industrial action at schools that are to be converted into academies, saying the academies programme put public assets into private ownership.

Hank Roberts, who has been campaigning against new academies in north London, said: "We have to use every weapon in our arsenal. It includes strike action and direct action."

A fringe meeting at the conference heard that ministers were also concerned about academies that did not recognise teachers' unions. Sir Bruce Liddington, the schools commissioner, said Jim Knight, the schools minister, had described those schools as being on a "list of shame".

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